This invention relates to sodium vapor lamps and in particular to a low pressure sodium vapor lamp in which the discharge tube is partially coated with metallic sodium to form an opaque-reflective tube inner surface which in turn defines an aperture through which light output is directed. This aperture area is coated with a conductive coating, transparent in the visible spectrum, to provide ohmic heating of the aperture to avoid sodium condensation thereon.
Low pressure sodium vapor lamps have several desirable properties for application as exposure lamps in xerographic photocopiers. The spectral output of sodium lamps is almost monochromatic, emitting essentially at 589.0 and 589.6 nanometers, this in the yellow portion of the spectrum. As has been pointed out in U.S. Pat. No. 3,869,205, this yellow radiation is particularly desirable for creating object contrast for copying of most documents and paper work in general use. Furthermore, the efficiency of low pressure sodium lamps is relatively high and compares favorably with other linear light sources.
For xerographic application, there are certain characteristics of sodium lamps that present problems to be overcome. Excess metallic sodium is generally present within the lamp discharge tube to control the vapor pressure within the tube. This metallic sodium is opaque and absorbs some of the radiation from the sodium vapor discharge and to that extent lowers the lamp efficiency. Also, for most xerographic applications, it is desirable to focus the light emission from the exposure lamps onto a rectangular slit to minimize lost light.
In the prior art, as for example U.S. Pat. No. 3,221,198 to Van der Wal et al, the coating of a sodium vapor lamp tube with tin oxide for the purpose of visible transmission and infrared reflection, this in turn for the purpose of heat conservation is known.
It is an object of the present invention to provide a sodium vapor discharge lamp with a combination of a reflective coating of excess sodium defining a light aperture to provide desired directionality of visible light emission.